ES2820311T3 - High strength hot dip galvanized steel sheet with excellent impact peel strength and corrosion resistance of the machined portion - Google Patents

Abstract

A high strength hot dip galvanized steel sheet with excellent impact resistance and corrosion resistance of the machined portion, comprising a layer of hot dip galvanized plating containing Fe: 0.01 to 6.9 % by mass, Al: from 0.01 to 1.0% by mass, the remainder being made up of Zn and unavoidable impurities in a base material of the steel sheet containing, C: from 0.05 to 0.4% by mass, Si: from 0.4 to 3.0% by mass, Mn: from 1.0 to 4.0% by mass, P: from 0.0001 to 0.1% by mass, S: from 0, 0001 to 0.01% by mass, Al: from 0.005 to 0.1% by mass, N: from 0.0005 to 0.01% by mass, and O: from 0.0001 to 0.01% by mass, optionally one type or two types of: Ti: from 0.001 to 0.15% by mass, and Nb: from 0.001 to 0.10% by mass, and additionally, optionally, one type or two types or more of: Mo: from 0.01 to 2.0% by mass, Cr: from 0.01 to 2.0% by mass, Ni: from 0.01 to 2.0% by mass, Cu: from 0.01 to 2.0% by mass, and B: from 0.0001 to 0.01% by mass, the rest being constituted by Fe and unavoidable impurities, and with a tensile strength of 590 MPa or more, wherein: the plating layer includes protruding alloy layers that are in contact with the base material of the steel sheet, the number density of the layers of Outgoing alloy is 4 pieces / mm or more per unit length of an interface between the base material of the steel sheet and the plating layer when viewed from a sectional direction, and the maximum diameter of the outgoing alloy layers in the interface is 100 µm or less; wherein the projecting alloy layer is one having a thickness of 2 µm or more, and where the Fe-Al phase does not form at the interface between the projecting alloy layer and the base material of the steel sheet; and wherein the base material of the steel sheet includes: a miniaturized layer that is directly in contact with the interface between the base material of the steel sheet and the plating layer; a decarburized layer that is in contact with the miniaturized layer and exists on an inner side of the base material of the steel sheet; and an inner layer other than the miniaturized layer and the decarburized layer, wherein the average thickness of the miniaturized layer is 0.1 to 5 μm, and the average grain diameter of a ferrite phase in the miniaturized layer is 0 , 1 to 3 μm, the average thickness of the decarburized layer is 10 to 200 μm, the average grain diameter of a ferrite phase in the decarburized layer is 5 to 30 μm, the average volume fraction of the ferrite phase in the decarburized layer is 70% or more, and the remaining structure is composed of austenite, bainite, martensite or perlite, the ratio Hv (surface) / Hv (core) between the average micro Vickers hardness of the decarburized layer Hv (surface ) and the average micro Vickers hardness of the inner layer Hv (core) is 0.3 to 0.8, and one type or two types or more of Si and Mn oxides are contained in the layers of the miniaturized layer, the decarburized layer and the projecting alloy layers, where the number density and thickness of the exiting alloy layers, the average thickness of the miniaturized layer, the average thickness of the decarburized layer, the average grain diameter and the average volume fraction of a ferrite phase in the decarburized layer and the oxides of Si and Mn are determined as indicated in the description.

Description

DESCRIPTION

High strength hot dip galvanized steel sheet with excellent impact peel strength and corrosion resistance of the machined portion

Technical field

The present invention relates to a high strength hot-dip galvanized steel sheet and, in more detail, relates to a plated steel sheet applicable for various purposes, for example, for an automotive reinforcing element such as sheet metal. High strength hot dip galvanized steel with excellent impact resistance and plating adhesiveness.

Background of the technique

Hot-dip galvanized steel sheet has been frequently used for automobiles, household appliances, building materials, and the like. Extremely high formability is required for an automotive steel sheet that is pressed into complicated configurations. In addition, since the demand for automobile rust behavior has increased in recent years, hot-dip galvanized steel sheet is frequently used in automotive steel sheets.

In recent years, there is a growing need for a high-strength steel sheet with excellent strength and ductility in view of reducing the weight of the vehicle body. For example, Related Patent Document 1 describes a steel sheet having a steel sheet structure where a ferrite phase, a bainite phase and an austenite phase are mixed. Furthermore, it is disclosed that this steel sheet is a steel sheet that makes use of a transformation-induced plasticity that exerts high ductility by transforming the retained austenite into martensite at the time of formation.

This type of steel sheet contains, for example, 0.05 to 0.4% by mass of C, 0.2 to 3.0% by mass of Si and 0.1 to 2.5% by mass of Mn, and forms a complex structure by controlling a temperature pattern during a cooling process after annealing in a two-phase region. There is a feature that the desired properties can be ensured without using expensive alloying elements. In recent years, there is an increasing need for a high strength hot dip galvanized steel sheet where the base material surface of the steel sheet is subjected to hot dip galvanized plating to ensure rust performance. even for the aforementioned high strength steel sheet.

There are more and more cases where high-strength steel sheet is used not only as a reinforcing element for an inner plate use but also as an outer surface element that can receive impacts from flying stones or obstacles during driving time. on the body of a vehicle. When the high strength steel sheet is applied to a complicated configuration item, high processability is required. High strength hot dip galvanized steel sheet is necessary to ensure high plating adhesiveness evaluated by severe evaluation methods such as ball impact test and pull cord test, in addition to bending test. normal 60 ° V, assuming cases in which the impacts of flying stones or obstacles are received during driving and the adhesiveness of the plating in a hard machining time.

Since cracks occur in plating and an iron base with respect to a portion that receives extremely severe machining, such as a 180 ° flex machining vertex part, it is likely that portion corrosion will occur even after subjecting it to a conversion treatment and an electrodeposit coating. Even slight corrosion can cause hydrogen penetration from the corroded part to increase the possibility of hydrogen embrittlement cracking, particularly when the base material is high-strength steel sheet.

When the high-strength steel sheet is subjected to galvanized plating in a continuous hot-dip galvanizing unit, the wettability of the plating is greatly decreased when the amount of Si in the steel sheet exceeds 0.3% by mass. Therefore, there is a problem that a non-plating deteriorates the appearance quality occurs in the case of a Sendzimir method using a normal plating bath containing Al. In addition, it was difficult to ensure the adhesiveness of the plating on the moment of impact or hard machining moment because plating adhesiveness is greatly reduced simultaneously.

This is said to be because an external oxide film containing Si- and Mn-containing oxides having low wettability relative to molten Zn is generated on the surface of a steel sheet with a reduction in annealing time.

As a means of solving this problem, Patent Document 2 proposes a method in which the steel sheet is preheated in an atmosphere at an air ratio of 0.9 to 1.2 to generate Fe oxide, and then Plating is carried out in a bath where Mn and Al are added after the oxide thickness is set to 500 Á or less in a reduction zone containing H ² . However, there have been problems since it is difficult to precisely control the thickness of the oxide, and that the conditional manufacturing interval in a real machine is narrow because several steel sheets containing various additive elements are passed through a real line. Additionally, although an effect of improving the wettability and the adhesiveness of the plating could be expected in a normal machining time, the effect of improving the adhesiveness of the plating at the time of impact or the time of hard machining was small.

Related Patent Document 3 describes a method that improves plating ability by supplying a specific plating to a lower layer as another non-plating suppression means. However, this method requires providing a new plating facility in a previous stage of an annealing furnace on a hot plating line, or previously performing a plating treatment on an electroplating line. Both cases cause a significant increase in manufacturing costs. Additionally, the effects of improving the adhesiveness of the plating at the time of impact or the time of hard machining and the corrosion resistance of the machined part were small.

Meanwhile, the Related Patent Document 4 describes a method for making a hot dip galvanized steel sheet without oxidizing Fe in the steel sheet by adjusting an oxygen potential in an annealing atmosphere at the time of annealing. In this method, easily oxidizable elements such as Si and Mn in steel are oxidized internally by controlling the oxygen potential of the atmosphere to suppress the formation of an external oxide film, thereby enhancing the ability to undergo plating. Although sufficient adhesiveness at the time of normal machining can be ensured by applying this method, the effects of improving the adhesiveness of the plating at impact or hard machining time and the corrosion resistance of the machined portion could not be expected. . Other previously proposed arrangements are described in JP 2007 211279A.

Prior Art Documents

Patent documents

Patent Document 1: Japanese Patent Publication Open for Public Inspection No. H05-59429

Patent Document 2: Japanese Patent Publication Open for Public Inspection No. H04-276057

Patent Document 3: Japanese Patent Publication Open for Public Inspection No. 2003-105514

Patent Document 4: Japanese Patent Publication No. 4718782

Description of the invention

Problems to be solved by the invention

The present invention is made taking into account the present situation mentioned above, and an object thereof is to provide a high strength hot dip galvanized steel sheet with excellent impact peel strength and corrosion resistance of the machined portion. .

Means to solve problems

The authors of the present invention have carried out intensive studies in order to solve the aforementioned problems. As a result, the present inventors found that the adhesiveness of plating at an impact moment or hard machining moment is remarkably improved by forming an alloy layer in a salient state on a plating layer of a galvanized steel sheet. High strength hot dip even when a steel sheet containing a large amount of Si and Mn is used as the original plating sheet. At the same time, the present inventors found that it is possible to remarkably suppress the appearance and spread of cracks originating from a base material and to penetrate a surface layer of the plating layer even in an extremely severe state of distortion, such as as a 180 ° bending machining vertex part controlling a structure on one side of the base material of the steel sheet into a three-layer structure of a miniaturized layer, a decarburized layer and an inner layer. The present inventors further found that there is a remarkable improvement effect of the corrosion resistance of the machined portion while maintaining the strength as high as 590 MPa when laying the plating layer and the base material of the steel sheet. to have the structure mentioned above, to complete the present invention.

The present invention was made on the basis of such knowledge, and the invention is defined in the appended claims.

Effect of the invention

A high-strength hot-dip galvanized steel sheet according to the present invention makes it possible to provide a high-strength alloyed hot-dip galvanized steel sheet capable of ensuring plating adhesiveness at a moment of impact or a moment of machining. hard, and with a excellent corrosion resistance in the machined portion even in an extremely severe machined portion, such as a 180 ° bending machining vertex part, and is extremely effective for the purposes of automobile internal and external plates and a high-end element. reinforcement, even if a high-strength steel sheet containing a large amount of Si and Mn is used as the original sheet.

Brief description of the drawings

[FIG. 1] FIG. 1 is a view illustrating an example of a schematic view of a cross-sectional structure of a high-strength hot-dip galvanized steel sheet of the present invention.

[FIG. 2] FIG. 2 is a cross-sectional photograph of an inner layer of a comparative example and a cross-sectional photograph of an inner layer of an example of the present invention.

Mode of carrying out the invention

Next, a high strength hot dip galvanized steel sheet with excellent impact peel strength and corrosion resistance of the machined portion and a manufacturing method thereof according to an embodiment of the present invention is described in detail.

A high strength hot dip galvanized steel sheet of this invention contains:

C: from 0.05 to 0.4% by mass,

Yes: 0.4 to 3.0% by mass,

Mn: 1.0 to 4.0% by mass,

P: 0.0001 to 0.1% by mass,

S: 0.0001 to 0.01% by mass,

Al: 0.005 to 0.1% by mass,

N: 0.0005 to 0.01% by mass, and

O: 0.0001 to 0.01% by mass,

the remainder being made up of Fe and unavoidable impurities,

and includes a layer of hot dip galvanized plating composed of Fe: 0.01 to 6.9% by mass, Al: 0.01 to 1.0% by mass, the remainder being Zn and unavoidable impurities on a steel sheet base material whose tensile strength is 590 MPa or more, where

The plating layer includes projecting alloy layers that are in contact with the base material of the steel sheet, and the number density of the projecting alloy layers is 4 pieces / mm or more per unit length of an interface between the base material of the steel sheet and the plating layer when viewed from a sectional direction, and the maximum diameter of the projecting alloy layers at the interface is 100 pm or less,

the base material of the steel sheet includes:

a miniaturized layer that is directly in contact with the interface between the base material of the steel sheet and the plating layer;

a decarburized layer that is in contact with the miniaturized layer and exists on an inner side of the base material of the steel sheet; Y

an inner layer other than the miniaturized layer and the decarburized layer, where

the average thickness of the miniaturized layer is 0.1 to 5 pm, and the average grain size of a ferrite phase in the miniaturized layer is 0.1 to 3 pm,

the average thickness of the decarburized layer is 10 to 200 pm, and the average grain size of a ferrite phase in the decarburized layer is 5 to 30 pm, and the average volume fraction of the ferrite phase in the decarburized layer is 70% or more,

the remaining structure is composed of austenite, bainite, martensite or pearlite,

the ratio Hv (surface) / Hv (core) between an average Vickers hardness of the decarburized layer Hv (surface) and an average Vickers hardness of the inner layer Hv (core) is 0.3 to 0.8, and

One type or two types or more of Si and Mn oxides are contained in the layers of the miniaturized layer, the decarburized layer and the projecting alloy layers.

In FIG. 1 illustrates a schematic cross-sectional view of the plating layer, miniaturized layer, decarburized layer, and inner layer in the high-strength hot-dip galvanized steel sheet of this invention.

"Protruding alloy layers in plating layer"

In a high strength hot-dip galvanized steel sheet of this invention, the adhesiveness of the plating at an impact moment or a hard machining moment can be ensured by including protruding alloy layers in a plating layer. Large depressions and protrusions can be formed at an interface between a steel sheet base material and the plating layer including protruding alloy layers 2 as illustrated in FIG. 1 in the plating layer, and a remarkable improvement effect of plating adhesiveness due to anchoring effect can be expected even when strong shear stress acts in an interface direction between the base material of the steel sheet and the plating layer when receiving impact and hard machining. As a mode of the protruding alloy layers 2, a greater anchoring effect can be expected in a mode in which small protruding alloy layers are dispersed instead of just forming thick protruding alloy layers. Accordingly, the effective anchoring effect cannot be expected when a maximum diameter of the alloy layers 2 protruding at the interface between a base material 4 and a plating layer 1 which is indicated by 3 in FIG. 1 is greater than 100 pm because it is too large. Therefore, an upper limit of a maximum length (the maximum diameter 3) of the projecting alloy layers is set at 100 pm. The upper limit is preferably set at 40 pm. Although the lower limit of the maximum length of the projecting alloy layers 2 is not particularly limited, it is preferably set at 3 pm or more. An effect of improving the adhesiveness is shown when a number density of the exiting alloy layers is set to 4 pieces or more by an interface length of 1mm between the base material of the steel sheet and the plating layer when the The interface between the base material of the steel sheet and the plating layer is viewed from a sectional direction. Meanwhile, when the number density of the protruding alloy layers is more than 100 pieces / mm, not only the effect is saturated, but also the chipping resistance can be deteriorated. The upper limit of the number density of the projecting alloy layers is desirably set at 100 pieces / mm. The number density is preferably set in a range of 10 to 60 pieces / mm. As illustrated in FIG. 1, the protruding alloy layers 2 are in contact with the interface between the base material 4 and the plating layer 1, and each has a structure entering from the interface to the plating layer 1 in a protruding state. The shape of each projecting alloy layer 2 is arbitrary as long as the projecting alloy layer 2 is in contact with the interface 3 and enters the plating layer 1. Since the projecting alloy layers 2 are in contact with the interface, With the base material 4 not being interrupted by an Fe-Al phase, and protruding into the plating layer 1, the adhesiveness of the plating is considered to be improved due to the anchoring effect.

The protruding alloy layers of this invention are formed by performing a light alloy heat treatment after immersion in a plating bath as described below. In the plating bath, columnar microcrystals (hereinafter referred to as the bath crystallized phase) of a microscopic and columnar Z phase (FeZn13) and 51 phase (FeZnz) that crystallize directly to form an interface between the base material of the steel sheet and molten zinc do not adversely affect the effects of this invention even if it exists together with the protruding alloy layers, but an effect of improving the adhesiveness at the moment of impact or at the moment of impact cannot be expected. hard machining. To distinguish between the exiting alloy layer and the crystallized phase in the bath, the exiting alloy layer is defined as one that has a thickness of 2 pm or more, and where the Fe-Al phase does not form at the interface between the salient alloy layer and the base material of the steel sheet. Although the upper limit of the thickness of the protruding alloy layer is not particularly limited, it is preferably set at 90% or less of the total thickness of the plating layer. The exiting alloy layer crystallizes directly to interface with the base material of the steel sheet, and the Fe-Al phase does not exist at the interface. The projecting alloy layer is considered to be effective in improving adhesiveness because the projecting alloy layer is in direct contact with the base material without being interrupted by the Fe-Al phase.

Although the types of phases that form the exiting alloy layer are not particularly limited, a single phase structure or a multiphase structure selected from Z phase (FeZn13), 51 phase (FeZnz), an r1 phase (Fe5Zn21), is more preferable. and an r phase (Fe3Zn10) which are phases of Fe-Zn-based intermetallic compounds.

"Measuring method of the protruding alloy layer"

There is a method of measuring the maximum length and the numerical density of the exiting alloy layers, in which a 0.5% nital etching is made after embedding and polishing a cross section, an image is photographed with an optical microscope with a 200-fold increase to find the number density per unit length. The maximum length of the projecting alloy layers is measured using the same photograph. The lengths of the protruding alloy layers are measured with respect to each of the five photographs taken at a magnification of 200 times as for a sample, and a maximum value between them is set to the maximum length of the layers of alloy protrusions in the sample.

Although the protruding alloy layers are generated from the interface between the plating layer and the base material of the steel sheet through an alloy reaction, the gloss of the surface is reduced and the appearance uniformity decreases when the Protruding alloy layers reach an upper surface of the plating layer. Therefore, it is more preferable that the protruding alloy layers do not exist on the upper surface of the hot-dip galvanized plating layer in the high-strength hot-dip galvanized steel sheet of this invention.

"Fe concentration of the plating layer"

As noted above, a mode control of the protruding alloy layers is important in the high strength hot dip galvanized plating layer of this invention. The protruding alloy layers can be included in the plating layer by setting an Fe concentration of 0.01% by mass or more. The partial alloying reaction continues to the surface of the plating layer and the effect of improving the adhesiveness of the plating becomes small when the Fe concentration is set above 6.9% by mass. Therefore, the Fe concentration in the plating layer is limited to a range of 0.01 to 6.9% by mass. The range is preferably 2.0 to 6.9% by mass.

"Al concentration of the plating layer"

When the concentration of Al in the plating layer is less than 0.01% by mass, an excessive reaction of Fe-Zn in the plating bath cannot be controlled, and the control of the structure of the plating layer becomes difficult. When the Al concentration is 1.0 mass%, there is a possibility that the spot weldability is obstructed because a dense Al2O3 film is formed on the surface of the plating layer. The concentration of Al in the plating layer is more preferably set from 0.03 mass% to 0.8 mass% in view of controlling the structure of the plating layer. Additionally, the Al concentration is preferably set in a range of 0.1% by mass to 0.5% by mass.

"Other unavoidable impurities"

In carrying out this invention, one type or two types or more of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, Sr, In, Cs , and REM can be contained or mixed in the hot dip galvanized plating layer. The effects of the present invention are not affected even if one type or two types or more of the above-mentioned elements are contained or mixed in the hot-dip galvanized plating layer, and there may be a preferable case so that it is improved. resistance to corrosion and workability depending on its content.

"Plating Composition Measurement Method"

The plating layer is melted in a 5% aqueous HCl solution with an added inhibitor, and the molten liquid is analyzed by ICP to quantify it in order to measure the Fe concentration and the Al concentration in the plating layer.

"Structure on the side of the steel sheet base material"

Next, a structure on the base material side of the steel sheet in the high-strength hot-dip galvanized steel sheet of this invention is described in detail.

"Miniaturized layer"

As illustrated in FIG. 1, The high-strength hot-dip galvanized steel sheet of this invention includes a miniaturized layer 5 that is directly in contact with the interface between the base material of the steel sheet and the plating layer on the side of the base material. of the steel sheet. A layer consisting mainly of extremely microscopic grains of a ferrite phase is formed in the miniaturized layer 5, and the appearance of cracks from inside the base material of the steel sheet and the extent of cracks after that can be suppressed even in a portion under an extremely severe deformed state such as a 180 ° bending machining vertex part.

An effect of suppressing the occurrence of cracks and spreading at the time of machining is shown when the average thickness of the miniaturized layer is set to 0.1 gm or more. When the average thickness of the miniaturized layer is set to more than 5 gm, the alloy excessively continues in the plating bath and the structure of the plating layer of this invention cannot be obtained. Accordingly, the average thickness of the miniaturized layer is limited to a range of 0.1 to 5 gm. The average thickness of the miniaturized layer is preferably set in a range of 0.1 to 3 gm. The effect of suppressing the occurrence and spread of cracks at the time of machining is shown when an average grain size of the ferrite phase in the miniaturized layer is set to 0.1 gm or more, and the effect becomes limited when the Average grain size is set at more than 3 gm. Consequently, the average grain size of the ferrite phase in the miniaturized layer is limited to a 0.1 to 3 gm range. The average grain size is preferably set in a range of 0.1 to 2 gm.

In this invention, annealing under a controlled condition to be a specific temperature zone and a specific atmosphere is performed in an annealing process to generate the miniaturized layer and the decarburized layer as described below. As a result, the decarburization reaction continues to a surface layer of steel sheet base material in the specific temperature zone. Since the base material of the steel sheet is decarburized in the miniaturized layer, a constituent phase in the miniaturized layer is substantially a structure whose main body is the ferrite phase except for oxide and inclusion particles.

In this invention, the effect obtained by including the miniaturized layer on the base material side of the steel sheet is to suppress the appearance and spread of cracks at the time of hard machining as described above. At the same time, there is an effect to accelerate an Fe-Zn alloy reaction between the base material of the steel sheet and the hot-dip galvanized plating layer during a heating and alloying treatment after the hot-dip galvanized plating. hot to form the protruding alloy layers by miniaturizing the ferrite grain size in the surface layer of the base material of the steel sheet. Therefore, it is possible to suppress an amount of input heat required for the formation of the outgoing alloy layers to be low and to make the heating temperature during the alloy treatment process low in the state in which it is included. the miniaturized layer. An Fe-Zn reaction rate decreases if the heating temperature is lowered during the alloy treatment process, and therefore it is easy to slow down the reaction before the exiting alloy layers cover the entire plating layer and can expand the manufacturable conditional interval.

"Miniaturized layer measurement method"

In order to measure the miniaturized layer, a cross section is processed using a CP (Cross Section Polisher) device, a reflected electron image taken by an FE-SEM (Field Emission Scanning Electron Microscopy) is observed with a magnification 5,000 times, to measure the average thickness of the miniaturized layer and the average grain size of the ferrite phase in the miniaturized layer. The miniaturized layer is defined to exist when an average grain size of the ferrite phase in the upper surface layer of the base material of the steel sheet is 1/2 or less of the average grain size of a ferrite phase in the layer. decarburized. An interface where the average grain size of the ferrite phase in the miniaturized layer becomes more than 1/2 of the average grain size of the ferrite phase in the decarburized layer is defined as a layer interface between the miniaturized layer and the decarburized layer.

"Decarburized layer"

In the high strength hot dip galvanized steel sheet of this invention, there is a decarburized layer 6 as illustrated in FIG. 1. In the decarburized layer 6, since a volume fraction of the hard phases (remaining structures 9) is lower and the resistance is also lower compared to an inner layer 7, it is unlikely that the decarburized layer 6 is the origin of Cracks even in severe distorted state in 180 ° bending machining vertex part, the occurrence of cracks in 180 ° bending machining vertex part can be suppressed. The effect of suppressing the occurrence of cracks even in the vertex part of the 180 ° flex machining is exerted by setting an average thickness of the decarburized layer at 10 gm or more, but the tensile strength of the entire base material of the Steel sheet is reduced due to the properties of the decarburized layer when the average thickness is set to more than 200 gm. Therefore, the average thickness is limited to a range of 10 to 200 gm. The range is preferably 30 to 150 gm.

"Steel sheet structure in the decarburized layer"

The decarburized layer 6 has a mixed structure where a ferrite phase 8 is a main body, and the remaining structures 9 are occupied by one type or two types or more from among the austenite phase, the bainite phase, the martensite phase. and the pearlite phase as illustrated in FIG. 1. The average hardness of the decarburized layer 6 relative to the inner layer 7 is sufficiently reduced by setting the volume fraction of the ferrite phase in the decarburized layer 6 to 70% or more, and therefore, the effect of suppressing the Crack appearance is shown in the 180 ° bending machining vertex part. When the average grain size of the ferrite phase in the decarburized layer is less than 5 gm, the softening effect of the decarburized layer is insufficient. When the average grain size of the ferrite phase in the decarburized layer is greater than 30 gm, there is a possibility that the low temperature toughness will deteriorate. The average grain size of the ferrite phase in the decarburized layer is therefore limited to a range of 5 to 30 gm. The ratio Hv (surface) / Hv (core) between an average Vickers hardness of the decarburized layer Hv (surface) and an average Vickers hardness of the inner layer Hv (core) can be set in a range from 0.3 to 0, 8 by the decarburized layer having the structure of this invention. It is necessary to reduce the hardness of the decarburized layer with respect to the hardness in the core to avoid the appearance of cracks in the vicinity of the interface between the base material of the steel sheet and the surface layer of the plating layer in the part. from the vertex of the 180 ° flex machining. When the Hv (surface) / Hv (core) is less than 0.3, there is a possibility that there will be a negative effect on the strength of the entire base material of the steel sheet because the hardness of the decarburized layer is too high. short. When the Hv (surface) / Hv (core) is greater than 0.8, cracks may occur in the machining vertex part of 180 ° bending because the decarburized layer is not sufficiently softened with respect to the inner layer. Therefore, the Hv (surface) / Hv (core) is limited to the range of 0.3 to 0.8 in this invention. The Hv (surface) / Hv (core) is preferably set in a range of 0.3 to 0.6.

"Measurement method of the decarburized layer"

To measure the thickness of the decarburized layer, first, the cross section of the steel sheet is embedded and polished, a hardness curve is measured using micro Vickers from the interface between the base material of the steel sheet and the plating layer. towards the side of the base material of the steel sheet, to find a thickness of a layer whose hardness is lower with respect to the hardness of the inner layer. The found thickness of the layer includes both the thickness of the decarburized layer and the thickness of the miniaturized layer, and the value at which the thickness of the miniaturized layer found through the above-mentioned method is subtracted from the thickness of the layer. found using the micro Vickers, it is the thickness of the decarburized layer. The average value of the hardnesses measured in the decarburized layer is set as Hv (surface), and the average value of the hardnesses measured in the inner layer is set as Hv (core).

To find the volume fraction of the ferrite phase in the decarburized layer, a sample is obtained while establishing a thickness cross section that is parallel to a rolling direction of the base material of the steel sheet as the observation surface, the surface Observation is polished and etched with nital, then observed by FE-SEM on the decarburized layer to measure an area fraction of the ferrite phase, and the result can be considered as the volume fraction. The grain size of the ferrite phase can be measured simultaneously.

"Structure of the inner layer"

The structure mode of the inner layer in this invention is not particularly limited as long as the tensile strength of the steel sheet is 590 MPa or more, and an Hv (surface) / Hv (core) can be ensured within the range from 0.3 to 0.8. However, the structure is preferably made up of 50% or more of the ferrite phase and the remaining structures 9 are composed of martensite, austenite, bainite and pearlite in order to ensure a balance between strength and ductility.

"Effect of improving the corrosion resistance of the machined portion"

The high strength hot-dip galvanized steel sheet of this invention includes the projecting alloy layers on the plating layer, and the miniaturized layer and the decarburized layer on the base material side of the steel sheet. The effect as a simple substance of each is as described above, but a remarkable improvement effect of corrosion resistance can be obtained which could not be expected in the past in a portion machined under extremely severe distorted state, such as the 180 ° flex machining vertex portion when all layers are provided as defined in this invention. When only the protruding alloy layers exist, but the miniaturized layer and decarburized layer do not exist in the surface layer of the base material of the steel sheet, cracks may occur because the distortion is large in the surface layer of the base material of the steel sheet. Steel sheet of the 180 ° bending machining vertex part, resulting in cracks penetrating the surface of the plating layer, and the corrosion resistance of the machined portion is lowered. When the miniaturized layer and the decarburized layer exist on the base material side of the steel sheet but the protruding alloy layers do not exist, the plating layer is deformed after a large distortion of the base material, the adhesiveness decreases markedly in the vicinity of the interface between the base material of the steel sheet and the plating layer, which results in the plating layer peeling off to reduce the corrosion resistance of the machined portion, although cracks in the surface layer of the Base material of the steel sheet can be suppressed in the 180 ° bending machining vertex part.

In this invention, cracks do not occur from the surface layer of the base material of the steel sheet in the 180 ° bending machining vertex part only in the state where there are all the protruding alloy layers, the miniaturized layer and the decarburized layer, and the adhesiveness does not decrease in the proximity of the interface between the base material of the steel sheet and the plating layer and the plating does not peel off even in a region where the plating layer is deformed after deformation of the base material of the steel sheet due to the anchoring effect due to the existence of the projecting alloy layers. Accordingly, the corrosion resistance of the machined portion can be remarkably improved.

"Oxide"

In the high strength hot-dip galvanized steel sheet of this invention, the oxides containing one type or two types or more of Si and Mn are contained in the layers of the miniaturized layer, the decarburized layer, and the alloy layers. outgoing. The types of oxides contained in the miniaturized layer, the decarburized layer and the projecting alloy layers are more preferably one type or two types or more selected from SiO ² , Mn2SiO4, MnSiO3, Fe2SiO4, FeSiO3 and MnO.

"Rust on protruding alloy layers"

The effect obtained by forming the protruding alloy layers 2 in the plating layer 1 is an improvement in the adhesiveness of the plating at the moment of impact or the moment of hard machining as described above. The protruding alloy layers are formed by forming internal oxides on the surface of the base material of the steel sheet in a specific temperature zone during the annealing of the base material of the steel sheet, and a slight heat treatment of the alloy is carried out after the hot dip galvanized plating as described below. Since the projecting alloy layers 2, as illustrated in FIG. 1 can be formed by the above-mentioned reaction, the protruding alloy layers inevitably contain oxides. The oxides contained in the projecting alloy layers more preferably have a maximum diameter of 0.05 to 0.4 µm and a number density of 20 to 100 pieces / µm.

"Rust in the miniaturized layer"

In this invention, the miniaturized layer 5 having the structure illustrated in FIG. 1 can be formed by forming the internal oxides in the base material of the steel sheet inside in the specific temperature zone in the annealing time, and suppressing the growth of ferrite phase crystals on the surface layer of the base material of the steel sheet due to internal oxide particles as described below. The decarburized layer inevitably contains oxides. The oxides contained in the decarburized layer more preferably have a maximum diameter of 0.01 pm to 0.2 pm and a number density of 20 to 100 pieces / pm2.

"Measurement of oxide"

To measure the presence / absence, identification of a type, a maximum diameter and a numerical density of an oxide layer, a cross section of a sheet of steel plated by FIB (focused ion beam) is processed to make a sample of thin film, the sample is then observed by FE-TEM (field emission transmission electron microscopy) at a magnification of 3,000 times. Five visual fields are photographed with respect to a sample, and an average value of the numerical densities of the oxides across all visual fields is established as the numerical density of the sample. The maximum value of the diameters of the oxides measured in all visual fields is set as the maximum diameter of the oxide in the sample.

"Chemical composition of the base material of the steel sheet"

A chemical composition of the base steel sheet forming the high strength hot-dip galvanized steel sheet in accordance with the embodiment of the present invention is described below.

C: C is an element that increases the strength of steel and it is effective that it is contained at 0.05% by mass or more. However, since the strength increases too much to reduce workability when contained in excess, an upper limit of 0.4% by mass is set. Preferably, the content is set in a range of 0.07 to 0.3% by mass in view of workability and weldability.

Yes: If it is an effective element capable of improving strength without reducing ductility, and it is effective if it is contained at 0.4% by mass or more. On the other hand, when C is added above 3.0% by mass, the strength increasing effect becomes saturated and the ductility decreases. In addition, the deterioration of the wettability of the plating is noticeable, greatly deteriorating the appearance. Therefore, an upper limit of 3.0% by mass is set. Preferably, the content is set in a range of 0.5 to 2.5% by mass.

Mn: Mn is an important element to allow high strength and is added at 1.0 mass% or more. However, since sheet cracks are likely to occur, and also spot weldability deteriorates when Mn is contained in more than 4.0% by mass, an upper limit of 4.0% is set in mass. Preferably, the content is set in a range of 1.5 to 3.5% by mass in view of strength and workability.

P: Since P is also an element that increases the strength of steel, but reduces workability, an upper limit of 0.1% by mass is set. A lower limit of 0.0001% by mass is set because a large refining cost is required to reduce the P content to less than 0.0001% by mass. The content is preferably set at 0.005 to 0.02% by mass based on a balance between strength, workability and cost.

S: S is an element that reduces the hot workability and corrosion resistance of steel. The upper limit of the content is set at 0.01% by mass because hot workability and corrosion resistance deteriorate when the content exceeds 0.01% by mass. A lower limit of 0.0001% by mass is set because it is economically disadvantageous to reduce the content to less than 0.0001% by mass. However, the content is preferably 0.001% by mass or more because a surface defect is likely to occur when the S content is reduced too much.

Al: It is necessary to add 0.005% by mass or more of Al as a deoxidizing element of the steel, and to suppress the refining of the grain of the hot rolled material by A1N and the thickening of the glass grains in a series of heat treatment procedures to improve the material. Since there is a possibility that the weldability is deteriorated when the content is more than 0.1% by mass, the content is set to 0.1% by mass or less.

The content is preferably 0.08% by mass or less in order to reduce a surface defect due to clumping of alumina.

N: Since N increases the strength of the steel but reduces the workability, an upper limit of 0.01% by mass is set. The upper limit is more preferably set at 0.005% by mass or less when extremely high workability is required. The lower the N content, the more preferable it is, but a lower limit of 0.0005% by mass is set because excessive cost is required to reduce the content to less than 0.0005% by mass.

O: Since O forms oxide and impairs ductility and stretch frangibility, it is necessary to suppress the content. The upper limit of the O content is set at 0.010% because the deterioration of the stretch frangibility becomes noticeable when the O content exceeds 0.010%. The O content is preferably 0.007% or less, and more preferably 0.005% or less. Although the effects of the present invention are shown without particularly setting a lower limit of the O content, the lower limit is set at 0.0001% because a large increase in manufacturing cost is accompanied by setting the O content of less. of 0.0001%. The O content is preferably 0.0003% or more, and more preferably 0.0005% or more.

In addition, the following items can be added to the base material of the steel sheet of the hot-dip galvanized steel sheet in accordance with the embodiment of this invention, as required.

Ti: Ti is an element that contributes to increasing the strength of the steel sheet by strengthening by precipitation, strengthening the fine grain by suppressing the growth of ferrite crystal grains, and strengthening dislocation by suppressing from recrystallization. However, when the Ti content exceeds 0.150%, the carbonitride precipitation increases and the formability deteriorates, and therefore the Ti content is preferably 0.150% or less. In view of formability, the Ti content is more preferably 0.080% or less. Although the effects of the present invention are shown without particularly setting a lower limit of the Ti content, the Ti content is preferably 0.001% or more to sufficiently obtain the effect of increasing the strength by adding Ti. To further increase the strength of the steel sheet, the Ti content is more preferably 0.010% or more.

Nb: Nb is an element that contributes to increasing the strength of the steel sheet by strengthening by precipitation, strengthening the fine grain by suppressing the growth of ferrite crystal grains, and strengthening the dislocation by suppressing from recrystallization. However, when the Nb content exceeds 0.100%, the precipitation of the carbonitride increases and the formability deteriorates, and therefore, the Nb content is preferably 0.100% or less. In view of formability, the Nb content is more preferably 0.050% or less. Although the effects of the present invention are shown without particularly setting a lower limit of the Nb content, the Nb content is preferably 0.001% or more to sufficiently obtain the effect of increasing the strength by adding Nb. To further increase the strength of the steel sheet, the Nb content is more preferably 0.010% or more.

Mo: Mo suppresses phase transformation at high temperature and is an effective element for increasing resistance, and can be added in place of part of C and / or Mn. When Mo content exceeds 2.00%, workability during hot machining is affected and productivity decreases. Therefore, the Mo content is preferably 2.00% or less, and more preferably 1.40% or less. Although the effects of the present invention are shown without particularly setting a lower limit of the Mo content, the Mo content is preferably 0.01% or more, and more preferably 0.10% or more to sufficiently obtain the effect of increased resistance by adding Mo.

Cr: Cr suppresses phase transformation at high temperature and is an effective element for increasing strength, and can be added in place of part of C and / or Mn. When the Cr content exceeds 2.00%, the workability during hot machining is affected and the productivity decreases, and therefore the Cr content is preferably 2.00% or less, and more preferably 1.40. % or less. Although the effects of the present invention are shown without particularly setting a lower limit of the Cr content, the Cr content is preferably 0.01% or more, and more preferably 0.10% or more to sufficiently obtain the effect of increased resistance by adding Cr.

Ni: Ni suppresses phase transformation at high temperature and is an effective element to increase resistance, and can be added in place of part of C and / or Mn. When the Ni content exceeds 2.00%, the weldability is deteriorated, and therefore the Ni content is preferably 2.00% or less, and more preferably 1.40% or less. Although the effects of the present invention are shown without particularly setting a lower limit of the Ni content, the Ni content is preferably 0.01% or more, and more preferably 0.10% or more to sufficiently obtain the effect of increased resistance by adding Ni.

Cu: Cu is an element that increases resistance by existing as fine particles in steel and can be added instead of part of C and / or Mn. When the Cu content exceeds 2.00%, the weldability is affected, and therefore the Cu content is preferably 2.00% or less, and more preferably 1.40% or less. Although the effects of the present invention are shown without particularly setting a lower limit of the Cu content, the content of Cu is preferably 0.01% or more, and more preferably 0.10% or more to sufficiently obtain the effect of increasing strength by adding Cu.

B: B suppresses phase transformation at high temperature and is an effective element to increase resistance, and can be added in place of part of C and / or Mn. When the B content exceeds 0.010%, the workability during hot machining is affected and the productivity decreases. Therefore, the content of B is preferably 0.010% or less. In view of productivity, the content of B is more preferably 0.006% or less. Although the effects of the present invention are shown without particularly setting a lower limit of the content of B, the content of B is preferably 0.0001% or more to sufficiently obtain the effect of increasing strength by adding B. For further increasing the strength, the content of B is more preferably 0.0005% or more.

"Manufacturing method"

Next, a method of manufacturing the high-strength hot-dip galvanized steel sheet of this invention is described. In the manufacturing method of the high strength hot-dip galvanized steel sheet of this invention, a sheet having the above-mentioned chemical composition is used as the original sheet, and hot rolling, cooling, winding, pickling and cold rolled. Subsequently, after the heating and annealing by CGL is performed, the resultant is dipped into a hot-dip galvanized plating bath to make the high-strength hot-dip galvanized steel sheet.

The plate to be hot rolled is not particularly limited, and a continuously cast plate or a plate produced by a fine modified plate or the like can be used. The manufacturing method is compatible with a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is done immediately after casting.

The hot rolling finishing temperature is not particularly limited, but is more preferably 850 to 970 ° C in order to ensure the press formability of the steel sheet. Although the cooling conditions and the winding temperature after hot rolling are not particularly limited, the winding temperature is preferably set to 750 ° C or less to prevent the variation of the material in the two parts of the ends of the coil. large and the deterioration of the pickling ability due to the increased thickness of the flake. Additionally, the winding temperature is preferably set at 550 ° C or more because edge cracking is likely to occur in the cold rolling time, and sheet fracture may occur in the extreme case if the temperature of winding is too low. After performing the normal pickling to remove a black flake, a reduction ratio in cold rolling time can be a normal condition, and the reduction ratio is more preferably set to 50% or more to obtain a maximum improvement effect. of workability. Meanwhile, since many cold rolling loads are needed to perform cold rolling with a reduction ratio greater than 85%, the reduction ratio is more preferably set to 85% or less.

Hot dip galvanized plating is done after cold rolling as described above. As an example of a preferable manufacturing method of the high strength hot-dip galvanized steel sheet of this invention, the atmosphere, when the steel sheet having the above-mentioned chemical composition is subjected to hot-dip galvanized plating, is established in an atmosphere containing H ² from 0.1 to 20% by volume, the remainder being constituted by N ² , H ² O, O ² and unavoidable impurities, and the atmosphere between 650 ° C at maximum heating temperature is set in an atmosphere satisfying -1.7 <log (PH ² ü / PH ² ) <-0.6, and heating by temperature rise is performed at an average temperature rise rate of 0.5 to 5 ° C / s, after that, annealing is done continuously, then cooling is done at 650 ° C at an average cooling rate of 0.1 to 200 ° C / s, cooling is done at 650 ° C to 500 ° C at an average cooling rate of 3 to 200 ° C / s, the steel sheet is immersed in a bath of ch galvanized surface under galvanized plating bath temperature conditions: 450 to 470 ° C, a temperature of the steel sheet in a plating bath at the time of entry: 430 to 500 ° C, then heated and alloyed to 400 to 440 ° C for 1 to 50 s, and then the steel sheet is cooled to room temperature.

Hot-dip galvanized plating is preferably performed in all reducing furnaces of a continuous hot-dip galvanizing facility. The atmosphere at the annealing time is established in an atmosphere containing H ² from 0.1 to 20% by volume, the remainder being constituted by N ² , H ² O, O ² and unavoidable impurities. When the hydrogen is less than 0.1% by volume, the oxide film existing on the surface layer of the steel sheet cannot be sufficiently reduced and the wettability of the plating cannot be ensured. Therefore, an amount of hydrogen in the reducing annealing atmosphere is set to 0.1% by volume or more. When the hydrogen in the reducing annealing atmosphere exceeds 20% by volume, the dew point (corresponding to a partial pressure of water vapor PH ²⁰ ) increases excessively, and it is necessary to install a unit to prevent dew condensation. Since the installation of a new unit results in an increase in the cost of production, the amount of hydrogen in the reducing annealing atmosphere is set to 20% by volume or less. The amount of hydrogen is more preferably 0.5% by volume or more and 15% by volume or less.

The atmosphere from the temperature of 650 ° C to the maximum heating temperature is set so that the atmosphere satisfies -1.7 <log (PH ² ü / PH ² ) <-0.6, and the heating by temperature increase is performed at an average temperature rise rate of 0.5 to 5 ° C / s, and therefore, the miniaturized layer 5 and the decarburized layer 6 of this invention are formed as illustrated in FIG. 1. Recrystallization of a steel sheet structure rarely starts in a temperature zone below 650 ° C. Recrystallization begins and the nucleated recrystallized grains gradually grow in a temperature zone of 650 ° C or more. By increasing the log (PH ²⁰ / PH ² ) of the atmosphere during annealing to prepare the atmosphere on the easy oxidation side in the temperature zone, Si and Mn in the base material of the steel sheet are internally oxidized in the surface layer of the base material of the steel sheet, and the internal oxide particles suppress the grain growth of the recrystallized grains of the base material of the steel sheet to form the fine recrystallized grains in the surface layer of the base material of the sheet of steel, and the miniaturized layer can be formed 5. A decarburization reaction occurs in the surface layer of the base material of the steel sheet simultaneously with the internal oxidation to increase the volume fraction of the ferrite phase in the surface layer of the base material of the steel sheet, and the decarburized layer can be formed 6. When the log (PH ²⁰ / PH ² ) of the atmosphere between the temperature of 650 ° C and the maximum temperature of heat The decarburization layer is less than -1.7, the miniaturized layer and the decarburized layer cannot be formed because Si and Mn are seldom internally oxidized and the decarburization reaction also does not continue in the surface layer of the steel sheet. When the log (PH ²⁰ / PH ² ) is greater than -0.6, the thickness of the decarburized layer becomes too great to have a negative effect on the strength of the entire base material of the steel sheet. Consequently, the log (PH ²⁰ / PH ² ) is preferably set in a range of -1.7 <log (PH ²⁰ / PH ² ) <-0.6. The log (PH ²⁰ / PH ² ) is more preferably -1.3 <log (PH ²⁰ / PH ² ) <-0.7. When the average temperature rise rate in this temperature zone is more than 5 ° C / s, the miniaturized layer cannot be obtained because recrystallization in the surface layer of the base material of the steel sheet proceeds before they are formed. internal rust particles. Additionally, the decarburized layer cannot be obtained because the time necessary for the decarburization reaction to proceed cannot be sufficiently ensured. Meanwhile, when the average temperature rise rate in this temperature zone is less than 0.5 ° C / s, the decarburization reaction progresses excessively and there is a possibility that the strength of the entire base material of the sheet metal steel decrease. Therefore, the average temperature rise rate between 650 ° C and the maximum heating temperature is preferably set in the range of 0.5 to 5 ° C / s. The average temperature rise rate is more preferably set in a range of 0.5 to 3 ° C / s.

The maximum heating temperature is not particularly limited, but it is preferably set in a range of 800 to 900 ° C because there is a possibility that the shape of the sheet at the time of passage of the sheet through the high temperature, becomes very faulty when the maximum heating temperature exceeds 900 ° C.

In this invention, annealing is performed continuously after heating by increasing temperature. The annealing time is not particularly limited, and the conditions can be set as necessary, but the annealing time is preferably set in a range of 1s 300s in view of the economic efficiency and the surface property of the sheet. The annealing time is more preferably set in a range of 30s 150s.

After completion of annealing, the steel sheet is cooled to an immersion temperature in a plating bath. The average cooling rate from the maximum heating temperature to 650 ° C is desirably set between 0.1 and 200 ° C / s. Cooling rate of less than 0.1 ° C / s is undesirable because productivity is greatly affected. The upper limit is preferably set at 200 ° C / s because an excessive increase in the cooling rate results in an increase in the cost of manufacture. The cooling rate from 650 to 500 ° C is preferably set from 3 to 200 ° C / s. When the cooling rate is too small, the austenite is transformed into a pearlite structure during the cooling process, and it becomes difficult to ensure the austenite volume ratio of 3% or more. Preferably, the lower limit is set at 3 ° C / s. Meanwhile, there is no problem regarding the material if the cooling rate increases, but an excessive increase in the cooling rate results in an increase in the manufacturing cost. Preferably, the upper limit is set at 200 ° C / s. A cooling method can be any method between roll cooling, air cooling, water cooling, and a method that combines these methods.

The plating bath temperature in the hot dip galvanized plating process is preferably set between 450 and 470 ° C. When the plating bath temperature is lower than 450 ° C, the bath temperature control becomes unstable and there is a possibility that the bath is partially solidified. When the bath temperature exceeds 470 ° C, the service life of units, such as a dip roller and a zinc pot, is shortened. Accordingly, the temperature of the bath of the galvanized plating bath is preferably set between 450 and 470 ° C.

The inlet temperature of the sheet of the steel sheet in the plating bath is preferably set between 430 and 500 ° C. When the inlet temperature of the sheet is less than 430 ° C, the temperature of the plating bath drops considerably and it becomes necessary to provide a large amount of heat to the plating bath to stabilize the bath temperature. Accordingly, the lower limit is preferably set at 430 ° C. When the inlet temperature of the sheet exceeds 500 ° C, the Fe-Zn alloy reaction in the bath cannot be controlled, and it is difficult to control the amount of adhesion. Accordingly, the upper limit is preferably set at 500 ° C.

Although the Al concentration in the plating bath is not particularly limited, the effective Al concentration (a total Al concentration in the bath - a total Fe concentration in the bath) is preferably set in a range of 0.03 to 0.8% by mass to form the protruding alloy layers in the plating layer.

After immersion in the plating bath, the steel sheet is preferably subjected to the heating and alloying treatment at 400 to 440 ° C for 1s to 50s, and then cooled to room temperature. In the heat and alloying treatment process, the low-temperature alloying allows a local alloying reaction to proceed, and therefore the projecting alloying layers can be formed on the high-strength hot-dip galvanized steel sheet. of this invention. Since the protruding alloy layers are not formed because the local alloy reaction rarely occurs when the alloy temperature is below 400 ° C, the lower limit is preferably set at 400 ° C. Since it is difficult to obtain the mode of the exiting alloy layers because the alloy reaction expands not locally but fully when the alloy temperature exceeds 440 ° C, the upper limit is preferably set at 440 ° C. The heating time is preferably set in the range of 1 s to 50 s because the protruding alloy layers are not formed when the heating time is less than 1 s, and the line length of an alloying furnace becomes too long when the warm-up time exceeds 50 s.

In this invention, the heating and alloying treatment is preferably stopped before the protruding alloy layers generated from the interface between the plating layer and the base material of the steel sheet reach the surface of the plating layer. The heating time that is required until the alloy reaction progresses fully to the surface in the temperature range of 400 to 440 ° C is desirably found in advance by passing the steel sheet having the same component as the metal sheet. steel to be manufactured. By retaining the steel sheet while heating for 10 to 80% of the heating time required for the entire alloy (alloy completion time) found in advance, it is possible to accurately fabricate the protruding alloy layers without reaching the surface of the plating layer.

Example 1

Examples of the present invention are described below. The conditions in the examples are conditional examples used to verify the feasibility and effects of the present invention, and the present invention is not limited to these conditional examples. The present invention may employ various conditions as long as an object of the present invention is achieved without departing from the scope of the present invention.

The plates having the compositions listed in Table 1 were heated from 1,150 to 1,250 ° C, hot rolled so that the finishing temperature became 850 to 970 ° C to have hot rolled steel strips with a thickness of 2.4 mm each. After pickling, the resultants were cold rolled to obtain cold rolled steel strips with a thickness of 1.0 mm each, then passed through the hot dip galvanizing line under the listed conditions. in Table 2 to make hot dip galvanized steel sheets of Examples 1 to 34.

Table 1

Table 2

Table 2 (continued)

Table 2 (continued)

The steel sheets of the respective examples made by the above-mentioned method were subjected to evaluation tests as described below, and the results thereof were illustrated in Tables 3-1,3-2.

An adhesion amount of the plating layer was found by a gravimetric method by melting the plating layer on an evaluation surface in hydrochloric acid with inhibitor. At the same time, Fe and Al in the molten liquid were quantified by ICP to measure Fe concentration and Al concentration in the plating layer.

As described above, the maximum length and number density of the protruding alloy layers in the plating layer were found by embedding the cross section and mirror polishing, dipping in a nital etcher at 0.5 mass% for 1 to 3 seconds for acid etching and observing with a light microscope at 200 times magnification.

As described above, the average thickness of the miniaturized layer of the base material of the steel sheet and the average grain size of the ferrite phase in the miniaturized layer were measured by performing the CP procedure of the cross section, the reflected electronic image obtained by FE-SEM was observed with a 5,000-fold increase.

As described above, the average thickness of the decarburized layer of the base material of the steel sheet was found by performing mirror polishing after the cross section was embedded, measuring the hardness curve using micro Vickers of the interface between the material. base of the steel sheet and the plating layer towards the side of the base material of the steel sheet, and subtracting the thickness of the miniaturized layer found in advance from the thickness of the layer whose hardness is reduced with respect to the hardness of the inner layer.

As described above, the average grain size of the ferrite phase in the decarburized layer and the average volume fraction of the ferrite phase in the decarburized layer were determined by etching with 3% nital after embedding and polishing the section. cross-sectional, and observing a secondary electron image of the FE-SEM with a 2,000-fold magnification in the decarburized layer.

As described above, the Hv (surface) / Hv (core) was found by embedding and polishing the cross section, and then measuring the average value of the micro Vickers hardness of the decarburized layer Hv (surface) and the average value of the micro Vickers hardness of the inner layer Hv (core), and calculating the ratio of the same.

As described above, the presence / absence, type, maximum diameter, and number density of oxides were found in each of the projecting alloy layers, miniaturized layer, and decarburized layer when manufacturing the thin film sample by performing the FIB procedure of the cross section of the plated steel sheet, then observing the sample using the FE-TEM with a magnification of 30,000 times.

Regarding tensile strength in a tensile test, test pieces No. 5 according to JIS Z 2201 were processed from the steel sheets of the respective examples, and each tensile strength (MPa) was measured according to the test method described in JIS Z 2241.

The plating adhesiveness assuming a conventional procedure was evaluated by a V bending test. A 60 ° V bending mold was used for the V bending test. The steel sheet was bent 60 ° so that a surface The evaluation test was turned to an inner bending side using a mold having a radius of curvature at the tip of 1 mm, a tape was adhered to the inner side of the bent portion, and then the tape was peeled off. Spraying property was evaluated from the peeling state of the plating layer which peeled off together with the tape. It was evaluated as or; no detachment, A: detached and x: with noticeable detachment, and o was evaluated as pass.

The plating adhesiveness that is assumed when the steel sheet is impacted and at the time of hard machining was evaluated by a ball impact test. In the ball impact test, a mold having a hemispherical tip part with a diameter of 25 mm and a weight of 3.2 kg was dropped onto the hot-dip galvanized steel sheet from a height of 60 cm. , and a protruding part of the deformed hot-dip galvanized steel sheet was observed with a magnifying glass and peeled off with a tape for evaluation. It was evaluated as © ©: no detachment or cracks, ©: there are small cracks locally but no problem, or: there are tiny cracks and detachments locally but no problem, A: there is a large detachment and there is a problem, and x: there is a noticeable shedding and there is a problem, and © ©, ©, or are evaluated as approved.

Corrosion resistance of the machined portion in the extremely severely machined portion was verified using a sample after a 0T flex test (close 180 ° contact flex). A portion of the bent outer vertex of the 0T bending was established as an evaluation portion, and the sample after the 0T bending was subjected to the conversion treatment and electrodeposition coating under the following conditions.

Conversion treatment: zinc phosphate treatment, adhesion amount 2.5g / m2

Electrodeposit Coating: Pb-free epoxy-based electrodeposit paint, 20gm film thickness After that, an accelerated corrosion test described in JASO-M609-91 was performed, to evaluate the number of cycles in which red oxide was generated from the bent vertex part 0T. The results were evaluated according to the following criteria, and © ©, ©, or were evaluated as approved. © ©: no red rust, white rust after 150 cycles have passed, ©: no red rust and light white rust is generated after 150 cycles, or: no red rust and white rust is generated light after 120 cycles have passed, A: red oxide is generated after 60 cycles, and x: red oxide is generated after 30 cycles.

In all examples of this invention, the adhesiveness of the plating at the time of hard machining and the corrosion resistance of the portion machined at the time of hard machining are at an acceptable level, as can be seen in Table 3. In All the comparative examples that do not satisfy the scope of the present invention, the adhesiveness of the plating at the time of hard machining and the corrosion resistance of the machined portion at the time of hard machining deteriorate. FIG. 2 illustrates a cross-sectional photograph of an inner layer of the comparative example that is estimated to correspond to the Experimental round number 8, and a cross-sectional photograph of an inner layer of the example of this invention that is estimated to correspond to the Experimental round number 13.

Explanation of Codes

1 layer plating

2 layer alloy protruding

3 measure the direction of the diameter of the outgoing alloy layer

4 base material of steel sheet

5 miniaturized layer

6 layer decarburized

7 inner layer

8 phase ferrite

9 remaining structure (any of austenite phase, bainite phase, martensite phase, pearlite phase)

Claims (7)

1. A high strength hot-dip galvanized steel sheet with excellent impact resistance and corrosion resistance of the machined portion, comprising a layer of hot-dip galvanized plating containing Fe: 0.01 to 6 , 9% by mass, Al: from 0.01 to 1.0% by mass, the remainder being made up of Zn and unavoidable impurities in a base material of the steel sheet containing, C: from 0.05 to 0.4% by mass, Yes: 0.4 to 3.0% by mass, Mn: 1.0 to 4.0% by mass, P: 0.0001 to 0.1% by mass, S: 0.0001 to 0.01% by mass, Al: 0.005 to 0.1% by mass, N: 0.0005 to 0.01% by mass, and O: 0.0001 to 0.01% by mass, optionally one type or two types of: Ti: 0.001 to 0.15% by mass, and Nb: from 0.001 to 0.10% by mass, and optionally additionally one type or two types or more of: Mo: 0.01 to 2.0% by mass, Cr: from 0.01 to 2.0% by mass, Ni: 0.01 to 2.0% by mass, Cu: 0.01 to 2.0% by mass, and B: 0.0001 to 0.01% by mass, the remainder being made up of Fe and unavoidable impurities, and with a tensile strength of 590 MPa or more, where: the plating layer includes projecting alloy layers that are in contact with the base material of the steel sheet, the number density of the projecting alloy layers is 4 pieces / mm or more per unit length of an interface between the base material of the steel sheet and the plating layer when viewed from a sectional direction, and the maximum diameter of the projecting alloy layers at the interface is 100 gm or less; wherein the projecting alloy layer is one having a thickness of 2 gm or more, and where the Fe-Al phase does not form at the interface between the projecting alloy layer and the base material of the steel sheet; and where the base material of the steel sheet includes: a miniaturized layer that is directly in contact with the interface between the base material of the steel sheet and the plating layer; a decarburized layer that is in contact with the miniaturized layer and exists on an inner side of the base material of the steel sheet; Y an inner layer other than the miniaturized layer and the decarburized layer, where the average thickness of the miniaturized layer is 0.1 to 5 gm, and the average grain diameter of a ferrite phase in the miniaturized layer is 0.1 to 3 gm, the average thickness of the decarburized layer is 10 to 200 gm, the average grain diameter of a ferrite phase in the decarburized layer is 5 to 30 gm, the average volumetric fraction of the ferrite phase in the decarburized layer is 70% or more, and the remaining structure is composed of austenite, bainite, martensite or pearlite, the Hv (surface) / Hv (core) ratio between the average micro Vickers hardness of the decarburized layer Hv (surface) and the average micro Vickers hardness of the inner layer Hv (core) is 0.3 to 0.8, and one type or two types or more of Si and Mn oxides are contained in the layers of the miniaturized layer, the decarburized layer and the projecting alloy layers, wherein the number density and thickness of the projecting alloy layers, the thickness Average miniaturized layer, average decarburized layer thickness, average grain diameter and average volume fraction of a ferrite phase in the decarburized layer and the oxides of Si and Mn are determined as indicated in the description. 2. The high strength hot dip galvanized steel sheet with excellent impact resistance and corrosion resistance of the machined portion according to claim 1, wherein the oxides contained in the layers of the miniaturized layer, the decarburized layer and the exiting alloy layers are of one type or two types or more from SiO ² , Mn2SiO4, MnSiO3, Fe2SiO4, FeSiO3, and MnO. 3. The high strength hot dip galvanized steel sheet with excellent impact resistance and corrosion resistance of the machined portion according to claim 1 or claim 2, wherein the maximum diameter of the oxides contained in the layers of Alloy protrusions is 0.05 to 0.4 pm, and the number density is 20 to 100 pieces / pm2. 4. The high-strength hot-dip galvanized steel sheet with excellent impact resistance and corrosion resistance of the machined portion according to any one of claims 1 to 3, wherein the maximum diameter of the oxides contained in the layer miniaturized is 0.01 to 0.2 pm, and the number density is 20 to 100 pieces / pm2. 5. The high strength hot dip galvanized steel sheet with excellent impact resistance and corrosion resistance of the machined portion according to any one of claims 1 to 4, wherein the projecting alloy layers do not exist on a surface top layer of hot dip galvanized plating. 6. The high strength hot dip galvanized steel sheet with excellent impact resistance and corrosion resistance of the machined portion according to any one of claims 1 to 5, wherein the base material of the steel sheet contains a type or two types of: Ti: 0.001 to 0.15% by mass, and Nb: 0.001 to 0.10% by mass. The high strength hot dip galvanized steel sheet with excellent impact resistance and corrosion resistance of the machined portion according to any one of claims 1 to 6, wherein the base material of the steel sheet contains a type or two types or more of: Mo: 0.01 to 2.0% by mass, Cr: from 0.01 to 2.0% by mass, Ni: 0.01 to 2.0% by mass, Cu: 0.01 to 2.0% by mass, and B: 0.0001 to 0.01% by mass.